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Date: Thu, 30 Nov 2023 15:11:19 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Arseniy Krasnov <avkrasnov@...utedevices.com>, 
	Stefan Hajnoczi <stefanha@...hat.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Bobby Eshleman <bobby.eshleman@...edance.com>, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kernel@...rdevices.ru, 
	oxffffaa@...il.com
Subject: Re: [PATCH net-next v5 2/3] virtio/vsock: send credit update during
 setting SO_RCVLOWAT

On Thu, Nov 30, 2023 at 08:58:58AM -0500, Michael S. Tsirkin wrote:
>On Thu, Nov 30, 2023 at 04:43:34PM +0300, Arseniy Krasnov wrote:
>>
>>
>> On 30.11.2023 16:42, Michael S. Tsirkin wrote:
>> > On Thu, Nov 30, 2023 at 04:08:39PM +0300, Arseniy Krasnov wrote:
>> >> Send credit update message when SO_RCVLOWAT is updated and it is bigger
>> >> than number of bytes in rx queue. It is needed, because 'poll()' will
>> >> wait until number of bytes in rx queue will be not smaller than
>> >> SO_RCVLOWAT, so kick sender to send more data. Otherwise mutual hungup
>> >> for tx/rx is possible: sender waits for free space and receiver is
>> >> waiting data in 'poll()'.
>> >>
>> >> Signed-off-by: Arseniy Krasnov <avkrasnov@...utedevices.com>
>> >> ---
>> >>  Changelog:
>> >>  v1 -> v2:
>> >>   * Update commit message by removing 'This patch adds XXX' manner.
>> >>   * Do not initialize 'send_update' variable - set it directly during
>> >>     first usage.
>> >>  v3 -> v4:
>> >>   * Fit comment in 'virtio_transport_notify_set_rcvlowat()' to 80 chars.
>> >>  v4 -> v5:
>> >>   * Do not change callbacks order in transport structures.
>> >>
>> >>  drivers/vhost/vsock.c                   |  1 +
>> >>  include/linux/virtio_vsock.h            |  1 +
>> >>  net/vmw_vsock/virtio_transport.c        |  1 +
>> >>  net/vmw_vsock/virtio_transport_common.c | 27 +++++++++++++++++++++++++
>> >>  net/vmw_vsock/vsock_loopback.c          |  1 +
>> >>  5 files changed, 31 insertions(+)
>> >>
>> >> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>> >> index f75731396b7e..4146f80db8ac 100644
>> >> --- a/drivers/vhost/vsock.c
>> >> +++ b/drivers/vhost/vsock.c
>> >> @@ -451,6 +451,7 @@ static struct virtio_transport vhost_transport = {
>> >>  		.notify_buffer_size       = virtio_transport_notify_buffer_size,
>> >>
>> >>  		.read_skb = virtio_transport_read_skb,
>> >> +		.notify_set_rcvlowat      = virtio_transport_notify_set_rcvlowat
>> >>  	},
>> >>
>> >>  	.send_pkt = vhost_transport_send_pkt,
>> >> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
>> >> index ebb3ce63d64d..c82089dee0c8 100644
>> >> --- a/include/linux/virtio_vsock.h
>> >> +++ b/include/linux/virtio_vsock.h
>> >> @@ -256,4 +256,5 @@ void virtio_transport_put_credit(struct virtio_vsock_sock *vvs, u32 credit);
>> >>  void virtio_transport_deliver_tap_pkt(struct sk_buff *skb);
>> >>  int virtio_transport_purge_skbs(void *vsk, struct sk_buff_head *list);
>> >>  int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t read_actor);
>> >> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk, int val);
>> >>  #endif /* _LINUX_VIRTIO_VSOCK_H */
>> >> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>> >> index af5bab1acee1..8007593a3a93 100644
>> >> --- a/net/vmw_vsock/virtio_transport.c
>> >> +++ b/net/vmw_vsock/virtio_transport.c
>> >> @@ -539,6 +539,7 @@ static struct virtio_transport virtio_transport = {
>> >>  		.notify_buffer_size       = virtio_transport_notify_buffer_size,
>> >>
>> >>  		.read_skb = virtio_transport_read_skb,
>> >> +		.notify_set_rcvlowat      = virtio_transport_notify_set_rcvlowat
>> >>  	},
>> >>
>> >>  	.send_pkt = virtio_transport_send_pkt,
>> >> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>> >> index f6dc896bf44c..1cb556ad4597 100644
>> >> --- a/net/vmw_vsock/virtio_transport_common.c
>> >> +++ b/net/vmw_vsock/virtio_transport_common.c
>> >> @@ -1684,6 +1684,33 @@ int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t recv_acto
>> >>  }
>> >>  EXPORT_SYMBOL_GPL(virtio_transport_read_skb);
>> >>
>> >> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk, 
>> >> int val)
>> >> +{
>> >> +	struct virtio_vsock_sock *vvs = vsk->trans;
>> >> +	bool send_update;
>> >> +
>> >> +	spin_lock_bh(&vvs->rx_lock);
>> >> +
>> >> +	/* If number of available bytes is less than new SO_RCVLOWAT value,
>> >> +	 * kick sender to send more data, because sender may sleep in 
>> >> its
>> >> +	 * 'send()' syscall waiting for enough space at our side.
>> >> +	 */
>> >> +	send_update = vvs->rx_bytes < val;
>> >> +
>> >> +	spin_unlock_bh(&vvs->rx_lock);
>> >> +
>> >> +	if (send_update) {
>> >> +		int err;
>> >> +
>> >> +		err = virtio_transport_send_credit_update(vsk);
>> >> +		if (err < 0)
>> >> +			return err;
>> >> +	}
>> >> +
>> >> +	return 0;
>> >> +}
>> >
>> >
>> > I find it strange that this will send a credit update
>> > even if nothing changed since this was called previously.
>> > I'm not sure whether this is a problem protocol-wise,
>> > but it certainly was not envisioned when the protocol was
>> > built. WDYT?
>>
>> >From virtio spec I found:
>>
>> It is also valid to send a VIRTIO_VSOCK_OP_CREDIT_UPDATE packet without previously receiving a
>> VIRTIO_VSOCK_OP_CREDIT_REQUEST packet. This allows communicating updates any time a change
>> in buffer space occurs.
>> So I guess there is no limitations to send such type of packet, e.g. it is not
>> required to be a reply for some another packet. Please, correct me if im wrong.
>>
>> Thanks, Arseniy
>
>
>Absolutely. My point was different - with this patch it is possible
>that you are not adding any credits at all since the previous
>VIRTIO_VSOCK_OP_CREDIT_UPDATE.

I think the problem we're solving here is that since as an optimization 
we avoid sending the update for every byte we consume, but we put a 
threshold, then we make sure we update the peer.

A credit update contains a snapshot and sending it the same as the 
previous one should not create any problem.

My doubt now is that we only do this when we set RCVLOWAT , should we 
also do something when we consume bytes to avoid the optimization we 
have?

Stefano


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